[0001] This invention relates to a process for producing a polyarylene sulfide (for example,
a polyphenylene sulfide) having a low content of heavy metals, excellent electrical
characteristics, and excellent heat stability.
[0002] Polyarylene sulfide, having excellent heat resistance and excellent chemical resistance,
has attracted special attention for use in electronic components and automotive parts.
Being moldable into engineering parts made of plastics, films, sheets, fibers, etc.,
polyarylene sulfide is widely used in fields where heat resistance is needed.
[0003] Japanese Patent Publication No. 3368/1970 discloses a process for producing polyarylene
sulfide wherein the reaction between p-dichlorobenzene and sodium sulfide is conducted
in an organic polar solvent such as N-methyl-2-pyrrolidone or the like. Sodium sulfide
containing 2.8 moles or 9 moles of water of hydration per mole of Na
2S is heated to free the water of hydration under reduced pressure or in N-methyl-2-pyrrolidone
solvent with the introduction of bubbles of nitrogen and then p-dichlorobenzene is
added thereto, followed by thermal polymerization.
[0004] This process, however, causes the corrosion of reaction vessels made of iron or stainless
steel, because highly corrosive sodium sulfide is maintained at high temperatures
in the dehydration stage, resulting in elution of heavy metals such as iron or the
like into sodium sulfide. Therefore, a significant amount of heavy metals remains
in the polymer produced by the reaction with p-dichlorobenzene, which unfavorably
causes a reduction in heat stability and electrical characteristics. It is also conceivable
to use titanium, chromium, molybdenum, or tungsten, or stainless steels having a high
content of these metals, as the constituent materials of a reaction vessel in order
to prevent heavy metals remaining in the polymer. However, these materials are undesirable
because of their low reliability at high temperatures and pressure and their high
cost.
[0005] It is therefore an object of the present invention to provide a process.for producing
a polyarylene sulfide having a low content of heavy metals, excellent electrical characteristics,
and excellent heat stability even when a reaction vessel, which is made of low cost
materials, such as stainless steel having high reliability at high temperature and
pressure is used.
[0006] Other objects and advantages of the present invention will become apparent to those
skilled in the art from the following description and disclosure.
[0007] The present invention relates to a process for producing a polyarylene sulfide, which
comprises mixing an alkali metal sulfide, water and an organic polar solvent in the
ratio of about 1:10 to 20:1 to 10 (molar ratio) and by heating the mixture at a temperature
of from an initial point of about 100
0C to an end point of about 180 - 230°C to remove water, the molar ratio of the alkali
sulfide to water thereby being in the range of about 1:0.9 to 2.0, and then thermally
polymerizing the resulting mixture with about 0.90 to about 1.1 moles of polyhalogenated
aromatic compound per mole of the alkali metal sulfide at a temperature of from about
180 to about 300°C.
[0008] When the water of hydration is removed from a highly corrosive alkali metal sulfide
by heating it in a mixture which includes a large excess of water and an organic polar
solvent, the highly corrosive alkali metal sulfide at low temperature may be converted
to a reaction mixture comprising an extremely low corrosive alkali metal sulfide,
water and the organic polar solvent, which causes barely any corrosion of the reaction
vessel in the subsequent dehydration at high temperature.
[0009] Alkali metal sulfides usable in the present invention include lithium sulfide, sodium
sulfide, potassium sulfide, and mixtures thereof. An alkali metal hydrosulfide and
an alkali metal hydroxide may be mixed to produce an alkali metal sulfide in situ.
[0010] Moreover, a small amount of alkali metal hydroxide may be added to react with a trace
amount of the alkali metal hydrosulfide and alkali metal thiosulfate which are contained
in the alkali metal sulfide.
[0011] The water referred to in the present invention is the total sum of water present
in the reaction system, including the water of hydration of the alkali metal sulfide
and the water produced by the reaction between an alkali metal hydrosulfide and an
alkali metal hydroxide. Moreover, 10 to 20 moles of water per mole of alkali metal
sulfide should be added to the system of reaction prior to the removal of water by
heating. Less than 10 moles of water unfavorably leads to a larger proportion of heavy
metals being contained in the resulting polymer. More than 20 moles of water requires
more energy to remove water, which is undesirable from the economic viewpoint.
[0012] Preferable organic polar solvents usable in the present invention are aprotic polar
solvents which are stable at high temperatures. Examples of them include amides and
ureas such as N,N-dimethylacetamide, N-ethyl-2-pyrrolidone, N-methyl-2-pyrrolidone,
hexamethylphosphoramide, tetramethylurea, 1,3-dimethyl-2-imidazolidinone or the like,
sulfones such as sulfolane, dimethyl sulfolane or the like, ketones such as methyl
phenyl ketone or the like, and mixtures thereof.
[0013] The amount of the solvents used should be in the range of 1 to 10 moles per mole
of alkali metal sulfide. Less than one mole of the solvent unfavorably leads to a
larger proportion of heavy metals in the resulting polymer. More than 10 moles of
the solvent is undesirable from the economic viewpoint.
[0014] Polyhalogenated aromatic compounds usable in the present invention are halogenated
aromatic compounds in which two or more halogen atoms are bonded directly to the aromatic
nucleus. Examples of them include p-dichlorobenzene, o-dichlorobenzene, m-dichlorobenzene,
dibromobenzene, diiodobenzene, trichlorobenzene, tribromobenzene, triiodobenzene,
tetrachlorobenzene, tetrabromobenzene, tetraiodobenzene, dichloronaphthalene, dibromonaphthalene,
diiodonaphthalene, dichlorobiphenyl, dibromobiphenyl, diiodobiphenyl, dichlorodiphenyl
sulfone, dibromodiphenyl sulfone, diiododiphenyl sulfone, dichloro- benzophenone,
dibromobenzophenone, diiodobenzophenone, dichlorodiphenyl ether, dibromodiphenyl ether,
dichlorodiphenyl sulfide, dibromodiphenyl sulfide or the like, and mixtures thereof.
p-Dichlorobenzene and a mixture of p-dichlorobenzene and a small amount of trichlorobenzene
are usually used.
[0015] The amount of the polyhalogenated aromatic compound used is usually in the range
of about 0.90 to about 1.1 moles per mole of an alkali metal sulfide. Deviation from
this range unfavorably caused the polymer to possess a reduced molecular weight.
[0016] An auxiliary catalyst such as organic alkali metal carboxylate may also be added
to produce a polymer having a higher molecular weight.
[0017] The removal of water by heating in the present invention is usually conducted at
a temperature of from an initial point of about 100°C to an end point of about 180
- 230°C. When the temperature is higher than 230°C, a significant amount of organic
polar solvent is undesirably removed together with water by heating.
[0018] The preferable molar ratio relative to water of the alkali metal sulfide after the
removal of water is normally in the range of 1:0.9 to 2.0.
[0019] Polymerization in the present invention is conducted ordinarily at 180 to 300°C,
preferable 200 to 270°C, for 0.5 to 20.hours with stirring. When the polymerization
temperature is lower than 180°C, the polymerization rate is considerably reduced.
When the polymerization temperature is higher than 300°C, the thermal decomposition
of the polymer may take place.
[0020] The polyarylene sulfide obtained according to the present invention can be subjected,
in heat-treated or untreated conditions, as it is, or after being mixed with a reinforcing
filler (e.g. glass fiber), a known inorganic filler (e.g. mica, talc, or silica),
a pigment, a flame- retardant, a stabilizer, or other polymers, to injection molding,
extrusion molding, etc. to prepare a shaped article, a film, a sheet, a pipe, a fiber,
etc.
Examples
[0021] The present invention will be explained more specifically below by way of Examples.
However, the present invention is in no way restricted to these Examples.
[0022] The content of iron in the polymers shown in the Examples and Comparative Examples
was measured by (a) subjecting about 0.5 g of a sample to wet decomposition with about
10 ml of sulfuric acid and about 10 ml of nitric acid in a quartz beaker, (b) adding
thereto deionized water to prepare a solution of a predetermined amount and then (c)
subjecting the solution to atomic absorption spectrometry.
[0023] The heat stability of the polymers was determined by visual inspection of the color
of polymers when heat-treated for 2 hours at 250°C.
Example 1
[0024] In a SUS 316, 15-liter autoclave were placed 14.8 moles of Na2s.2.8H2o, 121 moles
of water, and 44 moles of N-methyl-2-pyrrolidone. They were heated to 210°C with stirring,
whereby 2630 g of a distillate consisting mainly of water was removed. Then, the system
was cooled to 170°C and 14.8 moles of p-dichlorobenzene was added. The system was
heated to 250°C. Polymerization was conducted for 3 hours at that temperature. After
the completion of the polymerization, heating was conducted under reduced pressure
to remove the solvent, and the resulting polymer was washed with hot water three times,
subjected to repeated centrifuging, and then dried overnight by heating to give a
light- white powder. The yield, iron content, and heat stability of the polymers were
measured. The results are shown in Table 1.
Example.? 2
[0025] The procedure of Example 1 was repeated except that 151 moles of water were added.
The results are shown in Table 1.
Example 3
[0026] The procedure of Example 1 was repeated except that 14.8 moles of NaSH (25% aqueous
solution) and 14.8 moles of NaOH (48% aqueous solution) were added instead of Na
2S.2.8H
2O and no further water was added. The results are shown in Table 1.
Example 4
[0027] The procedure of Example 1 was repeated except that 14.73 moles of p-dichlorobenzene
and 0.07 mole of 1,2,4-trichlorobenzene were added. The results are shown in Table
1.
Comparative Example 1
[0028] The procedure of Example 1 was repeated except that no water was added. The results
are shown in Table 1.
Comparative Example 2
[0029] The procedure of Example 1 was repeated except that 14.8 moles of Na
2S·9H
2O were added instead of Na
2S·2.8H
2O and no water was added. The results are shown in Table 1.
Example 5
[0030] The procedure of Example 1 was repeated except that 16.28 moles of N-methyl-2-pyrrolidone
were added prior to the removal of water and then p-dichlorobenzene was added together
with 27.72 moles of N-methyl-2-pyrrolidone to conduct polymerization. The results
are shown in Table 1.
Comparative Example 3
[0031] The procedure of Example 1 was repeated except that 13.32 moles of N-methyl-2-pyrrolidone
were added prior to the removal of water and then p-dichlorobenzene was added together
with 30.68 moles of N-methyl-2-pyrrolidone to conduct polymerization. The results
are shown in Table 1.
[0032] Examples 1 to 4 and Comparative Examples 1 to 2 apparently show that a molar ratio
of H
20/Na
2S of more than 10 before the removal of water gives an extremely low content of iron
as compared with those having a molar ratio of less than 10, and further a relatively
small degree of discoloration and indicating excellent heat stability after the application
of heat.
[0033] Example 5 and Comparative Example 3 show that a molar ratio of N-methyl-2-pyrrolidone/Na
2S of less than 1.0 before the removal of water gives an extremely high content of
iron even when additional N-methyl-2-pyrrolidone was added prior to polymerization,
resulting in reduced heat stability. Therefore, the molar ratio of N-methyl-2-pyrrolidone/Na
2S before the removal of water needs to be 1.0 or more.
[0034] As is obvious from the above explanation, the present invention provides a polyarylene
sulfide having a low content of heavy metals and excellent heat stability.